Pressure standard device

ABSTRACT

The present invention embodies a calibration/testing device having a housing having a generally cylindrical recess, a piston guided within the recess via a hydrostatic/aerostatic bearing, a pressure chamber formed between the piston and the cylindrical recess, a conduit connected to the pressure chamber for transmitting an exact pressure prevailing in the chamber, and a regulating device for maintaining the piston in a precisely constant position with respect to height independent of varying the load on the piston. The constant position is maintained by the feeding or discharging of fluid to and from the pressure chamber. The pressure within the pressure chamber is defined by loading the piston with calibration weights. The present invention offers the advantage of a comparatively simple structure, while maintaining a high level of accuracy.

BACKGROUND OF THE INVENTION

The invention relates to a pressure standard device as set forth in theclassifying portion of the claim.

The book "Messen, Steuern und Regeln in der chemischen Technik" , editedby J. Hengstenberg, B. Sturm, O. Winkler, third edition, volume I, pages112 to 117, Berlin 1980, discloses piston manometers in which acylindrical piston engages through an opening into a closed oil-filledcylinder to which the pressure p is applied by way of a further opening.The vertically directed piston is loaded with weights. Problems arise inthat arrangement with regard to sealing of the piston in the opening sothat no friction may occur and at the same time no fluid can to escape.The piston is therefore usually caused to rotate prior to the pressurebeing applied, and that reduces the level of friction. The frictionbetween the piston and the cylinder can be further reduced by air orfluid being radially introduced under pressure into the clearance. Afurther problem arises when measuring pressure in gases when making thetransition from the fluid to the gas, in which respect specialprecautions must be taken if the gas involved is not an inert gas. Eventhen making the transition from the fluid to the gas gives rise toproblems as the position in respect to the height of the piston in thecylinder is not defined. It is therefore necessary to provide stops withrelation to excessive movement of the piston both in an upward directionand in a downward direction.

German laid-open application (DE-OS) No. 35 14 911 describes anotherpressure measuring device for measuring a pressure applied to a pistonby way of a gaseous medium, wherein the piston is guided in a cylinderin a practically friction-free manner in bearings by virtue of the factthat the bearings are supplied with a fluid under pressure, the pistonacting on a force-measuring means. The bearing fluid which escapes intothe cylinder chamber containing the gas is removed in a regulated mannerin that arrangement. In an alternative embodiment, a gas under pressureis supplied to the bearings, while bearing gas which issues into thecylinder chamber can be used for building up the pressure in a conduitwhich is connected to the cylinder chamber. As the force-measuring meansoperates practically without performing any travel movement, practicallyno movement of the piston also occurs and the position of the pistonplays no part.

The GB-A-2056098 discloses a standardized pressure instrument comprisinga cylinder and a piston guided therein the height position thereofhaving to be maintained constant irrespective of a varying loading ofthe piston in order to prevent a contact to the cylinder. In the lowerregion of the cylinder a pressure space is formed being connected to apressurized fluid via a duct and by loading of the piston with acalibration mass the pressure in the pressure space may be determined. Aconstant height of zero position, respectively, of the piston isdetermined by an indicating device and is maintained by supplying fluidto the pressure space.

However, the guidance of the piston by means of upper and lowerdiaphragms having the piston tightly clamped therebetween isdisadvantageous. In view of this type of support the readings arefalsified due to the deformation resistance of the diaphragms anddeteriorate the measuring accuracy of the force measuring device. Inview of the diaphragms fixedly clamping the piston the latter may moveminimal only such that the height regulation for maintaining apredetermined height position must be designed extremely accurate.Furthermore, the expenditures for these diaphragms are considerable.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a calibration/testingdevice, the structure of which is considerably simplified, whilemaintaining a high level of accuracy, in comparison with known devices.

In accordance with the invention that object is attained by acalibration/testing device comprising a housing having a generallycylindrical recess, a piston guided within the recess viahydrostatic/aerostatic bearing means, a pressure chamber formed betweenthe piston and the cylindrical recess, a conduit connected to thepressure chamber for transmitting an exact pressure prevailing in thechamber, and a regulating device for maintaining the piston in aprecisely constant position with respect to height independent ofvarying the load on the piston. The constant position is maintained bythe feeding or discharging of fluid to and from the pressure chamber.The pressure within the pressure chamber is defined by loading thepiston with calibration weights.

Preferred developments of the device according to the invention arecharacterised in the subsidiary claims.

The hydrostatic or aerostatic bearing arrangement for the piston in thecylinder is practically friction-less so that there is no need forrotary movement of the piston. It is possible to use a liquid as thebearing fluid, by virtue of the regulated discharge thereof. Thearrangement is a particularly simple one when the bearing fluid is usedto build up the pressure. The level regulation effect which is used inthe invention provides for an exact position in respect of height of thepiston in the cylinder so that it does not bear against the end of thecylinder nor can it be displaced upwardly out of its bearings.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the calibration/testing deviceaccording to the invention will be apparent from the description of thespecific embodiments with reference to the following drawing wherein:

FIG. 1 is a partial sectional view illustrating the principle of a gaspressure standard device;

FIG. 2 is a practical embodiment of a gas pressure standard device basedon the embodiment shown in FIG. 1; and

FIG. 3 is a particularly simplified construction of a further embodimentof a gas or liquid pressure standard device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a gas pressure standard device 10 in accordance with afirst embodiment of the invention comprising a cylinder 12 which issupported on a base surface and in which a piston 14 is guided in apractically friction-less manner. The piston 14 can be loaded with acalibration weight 46. Formed in the interior of the cylinder 12 is apressure chamber 30 which is filled with gas, in particular air, andwhich by way of a conduit 48 applies a pressure p to a piece ofequipment which is to be calibrated or tested, for example a pressuremeasuring cell (see FIGS. 2 and 3). The pressure is essentiallydetermined by the calibration weight 46 and the area of the lower endface 16 of the piston 14.

The piston 14 is guided in the cylinder 12 in a practicallyfriction-less manner by way of hydrostatic bearings 20, as are known forexample from German laid-open applications (DE-OS) Nos. 31 43 919 or 3514 911. For that purpose, a bearing liquid, for example an oil, issupplied under pressure from a closed tank 22 by way of a pump 24 and afeed conduit 26. A part of the bearing liquid accumulates in pockets 44.However bearing liquid also passes into the pressure chamber 30 which isbeneath the piston surface 16 and which with the passage of time wouldbe filled up with the bearing liquid, which would lead to themeasurement result being adversely affected. The arrangement thereforeprovides for a controlled return of that bearing liquid from thepressure chamber 30. For that purpose a conduit 32 leads from thepressure chamber 30 into a closed catch tank 34 which is thus under thesame pressure as the pressure chamber 30. The amount of bearing liquidin the tank 34 is now held constant at a predetermined level. For thatpurpose, the tank has a level-detecting device 36 which can be, forexample, in the form of a float, whose position is preferably opticallyor electrically detected. When a given level is exceeded, a regulatingdevice 38 is activated, which opens an emptying valve 42, preferably byway of a motor 40, until the level of bearing liquid has fallen againbelow the reference level. The bearing liquid flows back into the tank22, thus providing a closed system in which there is no need forrefilling it with bearing liquid.

By virtue of the regulated discharge of bearing liquid from the pressurechamber 30, the volume which is under pressure does not change. Thus,the arrangement provides a high degree of accuracy. It should be notedthat the conduit 48 projects somewhat beyond the bottom end of thecylinder 12 so that no bearing liquid can penetrate into the conduit 48.

By virtue of different calibration weights 46 being put onto the device,exactly defined pressures are produced and applied to the test item, forexample a pressure-measuring cell, the display values of which thenserve to plot the usual calibration or test curve.

As the principle of the device according to the invention has beendescribed with reference to FIG. 1, reference will now be directed toFIG. 2 which is a diagrammatic view of an embodiment of acalibration/testing device 50 according to the invention, whichapproximates to a practical construction. The elements already describedwith reference to FIG. 1 are denoted by the same reference numerals andwill not be described in greater detail in this context.

In addition to the embodiment shown in FIG. 1, the calibration ortesting device shown in FIG. 2 has on the one hand a weight compensatingmeans 54 and on the other hand a position regulating means 70.

The compensating means 54 serves in particular to compensate for theweight of the piston 14. Although various devices can be considered forthat purpose, the construction which has been selected as an examplethereof comprises a two-armed lever 56 which is mounted at a stationarylocation and which at its one end is pivotally connected to the piston14 by way of a generally U-shaped member 52 while disposed at its otherend is a compensating weight 58 which, in the absence of a calibrationweight 46, precisely produces a condition of equilibrium.

FIG. 2 also shows the pressure-measuring cell 60 which is to becalibrated or tested and which is subjected with the pressure in theconduit 48. To provide a basic setting, gas, in particular air, can besupplied from a pump 64 which is driven for example by means of a motorM, until reaching a pressure which can be set by means of the adjustingmember 62. Upon an increase in the magnitude of the calibration weights46, the piston 14 would be urged into the cylinder 12 until finally itcomes to bear against the connecting feed portion of the conduit 48. Inorder to prevent that from happening and in order to maintain the piston14 in an exact position in the cylinder 12, provided at an upper edge ofthe piston 14 is a passive marking or an optical, magnetic or electricalsender 80 which is sensed by means of a sensor 78 which is mounted on aprojection at the upper edge of the cylinder 12, in oppositerelationship to the sender 80. The sensor 78 is provided with aregulating device which by way of an electric line 76 can actuate amotor 74 in one direction of rotation or the other, depending on whetherthe piston 14 is too high or too low. The motor 74 moves a controlmember 72 which supplies gas, in particular air, by way of a pressureconduit 68 to the conduit 48 until the piston 14 has reached its preciseposition, that is to say a zero position. In that way, upon analteration in the calibration weight 46, the piston 14 is automaticallyadjusted again into the correct position.

It should be pointed out that an automatic regulating means 50 is shownin the illustrated embodiment. It is however also possible to providefor positioning adjustment of the piston 14 by manual actuation of thecontrol member 72.

FIG. 3 shows a further embodiment of the invention which is of aparticularly simple design configuration. This embodiment makes use ofthe basic concept that the bearing fluid to provide for hydrostatic oraerostatic support for the piston 14 in the cylinder 12 is used at thesame time as a pressure medium. In this case the bearing fluid may be agas or a liquid, gas having the advantage that it does not have anadverse chemical effect on the pressure-measuring cell 60.

As in this embodiment no bearing liquid has to be discharged in aregulated fashion, the components used for that purpose in FIGS. 1 and 2are omitted, which makes the construction considerably simpler.

In this embodiment of a calibration or testing device 90, a given basepressure is applied to the bearing locations by way of the motor-drivenpump 24. By virtue of the bearing fluid leaking through the hydrostaticor aerostatic bearings, the pressure chamber 30 is filled with bearingfluid so that finally a corresponding pressure is applied by way of theconduit 48 to the test item, in particular the pressure-measuring cell60.

Precise positioning of the piston 14 is again achieved by way of theregulating means 70.

It should be pointed out that this embodiment can also use a weightcompensating means 54 as shown in FIG. 2.

Finally it should be pointed out that it is not sufficient for air to beinjected under pressure into a smooth oil bearing in order to preventcontact between the piston and the cylinder wall or the opening thereof.Just a minor asymmetry in respect of a calibration weight 46 which isput onto the device can result in the piston bearing against thecylinder wall.

As FIGS. 1 through 3 show, the hydrostatic or aerostatic bearingarrangement according to the invention comprises two spaced-apartbearing rings 20a and 20b, between which there is an annular pocket 44which is pressure-free, for example by virtue of a bore 45 (see FIG. 3).

As indicated in FIG. 1, the outlets of the conduit 26 to the bearingrings 20a and 20b respectively are provided with throttles which retarda feed and return flow of bearing fluid. That means that, in the eventof the piston 14 possibly assuming an inclined position, the gap betweenthe piston and the bearing channel becomes asymmetric, in which case ahigher pressure occurs at the narrower location and a lower pressureoccurs at the wider location, thus moving the piston back into avertical position.

In order further to improve the hydrostatic bearing arrangement, eachbearing channel 20a and 20b may again be divided into three mutuallyparallel individual channels or grooves which are respectively fed byway of throttles.

Although a specific form of embodiment of the instant invention has beendescribed above and illustrated in the accompanying drawings in order tobe more clearly understood, the above description is made by way ofexample and not as a limitation to the scope of the instant invention.It is contemplated that various modifications apparent to one ofordinary skill in the art could be made without departing from the scopeof the invention which is to be determined by the following claims.

I claim:
 1. A pressure standard device, comprising:a housing having agenerally cylindrical recess with an upper opening and a bottom wallopposite thereto; a piston guided with a minimum amount of friction insaid cylindrical recess operating by way of at least one of ahydrostatic and aerostatic bearing means; a pressure chamber formedbetween an inner end face of said piston and said bottom wall of saidcylindrical recess; a conduit means, connected to said pressure chamber,for transmitting an exact pressure prevailing therein, said pressurebeing precisely defined by loading said piston with calibration weights;and a regulating device maintaining said piston in a precisely constantheight position independent of varying loading of said piston byselectively feeding or discharging of fluid to and from said pressurechamber.
 2. A pressure standard device of claim 1, wherein a weightcompensating means is provided for compensating a weight of said piston.3. A pressure standard device of claim 2, wherein the bearing fluidentering said pressure chamber is removed in a controlled manner.
 4. Apressure standard device of claim 3, wherein said pressure chambercommunicates by way of a further conduit with a collecting tank intowhich the bearing fluid passing into said pressure chamber drains andwherein a level of the bearing fluid in said collecting tank is keptconstant by draining the bearing fluid in a controlled manner.
 5. Apressure standard device of claim 4, wherein a closed supply tank forthe bearing fluid is provided, from which the bearing fluid is pumpedinto said bearing means and into which the bearing fluid removed fromsaid pressure chamber is drained in a controlled manner.
 6. A pressurestandard device of claim 3, wherein an emptying valve is regulated inrespect of its transmission capability by way of a level display means.7. A pressure standard device of claim 3, wherein a closed supply tankfor the bearing fluid is provided, from which the bearing fluid ispumped into said bearing means and into which the bearing fluid removedfrom said pressure chamber is drained in a controlled manner.
 8. Apressure standard device of claim 2, wherein said regulating means has aheight-position sensor including a regulating means which is disposed onan output side thereof and which energizes a control member forselectively feeding or discharging of the pressure fluid to and fromsaid conduit.
 9. A pressure standard device of claim 1, wherein thebearing fluid entering said pressure chamber is removed in a controlledmanner.
 10. A pressure standard device of claim 9, wherein said pressurechamber communicates by way of a further conduit with a collecting tankinto which the bearing fluid passing into said pressure chamber drainsand wherein a level of the bearing fluid in said collecting tank is keptconstant by draining the bearing fluid in a controlled manner.
 11. Apressure standard device of claim 10, wherein a closed supply tank forthe bearing fluid is provided, from which the bearing fluid is pumpedinto said bearing means and into which the bearing fluid removed fromsaid pressure chamber is drained in a controlled manner.
 12. A pressurestandard device of claim 10, wherein said regulating means has aheight-position sensor including a regulating means which is disposed onan output side thereof and which energizes a control member forselectively feeding or discharging of the pressure fluid to and fromsaid conduit.
 13. A pressure standard device of claim 10, wherein aregulated pressure supply device is connected to said conduit forbuilding up an initial pressure.
 14. A pressure standard device of claim9, wherein an emptying valve is regulated in respect of its transmissioncapability by way of a level display means.
 15. A pressure standarddevice of claim 9, wherein a closed supply tank for the bearing fluid isprovided, from which the bearing fluid is pumped into said bearing meansand into which the bearing fluid removed from said pressure chamber isdrained in a controlled manner.
 16. A pressure standard device of claim15, wherein a regulated pressure supply device is connected to saidconduit for building up an initial pressure.
 17. A pressure standarddevice of claim 9, wherein said regulating means has a height-positionsensor including a regulating means which is disposed on an output sidethereof and which energizes a control member for selectively feeding, ordischarging of the pressure fluid to and from said conduit.
 18. Apressure standard device of claim 9, wherein a regulated pressure supplydevice is connected to said conduit for building up an initial pressure.19. A pressure standard device of claim 1, wherein said conduit projectsabove said bottom of said cylindrical recess.
 20. A pressure standarddevice of claim 1, wherein said regulating means has a height-positionsensor including a regulating means which is disposed on an output sidethereof and which energizes a control member for selectively feeding ordischarging of the pressure fluid to and from said conduit.
 21. Apressure standard device of claim 20, wherein a regulated pressuresupply device is connected to said conduit for building up an initialpressure.
 22. A pressure standard device of claim 1, wherein a regulatedpressure supply device is connected to said conduit for building up aninitial pressure.
 23. A pressure standard device of claim 1, whereinsaid bearing means comprise two spaced-apart circular bearing rows whichare separated from each other by an annular pocket which is essentiallypressure-free.